Technical Field
The present invention relates to a photovoltaic material and a photovoltaic device comprising the photoactive material arranged between a hole transport layer and an electron acceptor layer.
The present invention also relates to the use of the photovoltaic material.
Description of Related Arts
Thin film solar cell technology has relied until now on the use of ternary and quaternary semiconductor compounds that are based on copper indium gallium sulfide/selenide or copper zinc tin sulfide/selenide that have the chalcopyrite or kesterite crystal structure as described in the article, Jackson, P., et al., “New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells beyond 20%.” Progress in Photovoltaics: Research and Applications, Volume 19, Issue 7 (November 2011): pages 894-897.
A new class of solar cell devices based on colloidal nanocrystals have also been developed and demonstrated that mainly focus on binary semiconductors of PbS(Se) quantum dots (QDs) or CdSe QDs, nanorods or tetrapods. The article, Huynh, Wendy U., et al., “Hybrid Nanorod-Polymer Solar Cells.” Science 295, 2425 (2002), describes a hybrid nanorod-polymer solar cell in which nanorods are combined with the conjugated polymer poly-3 (hexythiophene) to create charge transfer junctions with high interfacial area.
Of particular interest and promise is the case of PbS QDs that have now reached compelling power conversion efficiencies on the order of 8% in view of the advances made on increasing carrier mobility via ligand exchange strategies using bidentate molecules that crosslink the quantum dots or by using atomic ligands based on halides that also passivate the surface of the quantum dots as well as facilitate physical attachment. For example, this is described in Ip, et al., “Hybrid passivated colloidal quantum dot solids.” Nature Nanotechnology, vol. 7 (2013): pages 577-582.
The structures employed in these materials consist of the depleted heterojunction in which a depletion region is formed between PbS QDs and a transparent electrode, typically ZnO or TiO2 or other n-type semiconductors such as CdS or Bi2S3 nanocrystals. The resultant structure therefore forms a p-n heterojunction between the p-type QDs and the n-type phase.
A major drawback of this technology is the fact that relies on toxic elements and therefore imposes regulatory concerns about large scale deployment of this technology.
There has been increasing interest recently in the development of non-toxic materials which has led to the demonstration of a variety of nanocrystals that do not contain Pb or Cd and their employment in solar cell devices.
These materials include Cu2S, CZTS, CI(G)Se, and Cu2O, which share a common feature: they are p-doped semiconductors and therefore not appropriate for use as electron acceptors in polymer-nanocrystal solar cells. Recently another non-toxic material was added to the list of the promising materials for solar harnessing that consists of the binary compound of Bi2S3 nanocrystals. This is material that has been shown to act as an n-type semiconductor due to sulfur vacancies in the crystal structure. The high doping associated with this mechanism has thus far impeded the achievement of high performance in solar cell devices. Therefore devices using this material as the n-type electron acceptor with donor polymers have shown efficiencies on the order of 1%.
The patent application WO2013153024 describes a photovoltaic nanocomposite and solar cell device including the photovoltaic nanocomposite, where the photovoltaic nanocomposite includes a film of solution processed semiconductor materials having an n-type material selected from n-type quantum dots and n-type nanocrystals, and a p-type material selected from p-type quantum dots and p-type nanocrystals, and where the n-type material has a conduction band level at least equal, compared to vacuum level, to that of the p-type material, the p-type material has a valence band at the most equal, compared to vacuum level, to that of the n-type material. In one example the n-type nanocrystal is PbS and the p-type material Bi2S3.
Thus, from what is known in the art, the development of a photovoltaic material with no toxic element and a good efficiency is still of great interest.